1. Field of the Invention
The present invention relates to an optically compensatory film adapted for forming a good-visibility liquid-crystal display device, exhibiting stable birefringence characteristic to an external stimulus such as heat and humidity, and excellent in weight saving, and a polarizing plate laminated with the optically compensatory film.
The present application is based on Japanese Patent Applications No. Hei. 11-258287 and 11-269058, which are incorporated herein by reference.
2. Description of the Related Art
Heretofore, there is known an optically compensatory film having a cellulose film base, and a gradient orientation layer of a discotic liquid-crystal polymer provided on the base. The film has an advantage that phase difference characteristic based on a gradient of its main refractive index in an in-plane direction with respect to a surface of the base compensates for the phase difference between liquid-crystal cells to thereby enlarge a viewing angle range permitting good visibility.
The birefringence characteristic of the optically compensatory film was, however, apt to be changed by an external stimulus such as heat and humidity. Hence, there was a problem that the compensating effect of the optically compensatory film became partially different to cause distortion in display and that the weight per unit area of the optically compensatory film was heavy. The increase of screen size has been required of a liquid-crystal display device, or the like. In the case, the heavy weight of the optically compensatory film-was a serious worry.
An object of the present invention is to develop an optically compensatory film exhibiting birefringence characteristic hardly changed by an external stimulus such as heat and humidity, excellent in stability of the birefringence characteristic and excellent in weight saving per unit area.
According to a first aspect of the present invention, there is provided an optically compensatory film comprising a transparent film base, and a birefringent phase retarder layer adhesively supported by said transparent film base, said transparent film base having a specific gravity of not larger than 1.15 and having an photoelastic coefficient of not larger than 30xc3x9710xe2x88x9212 m2/N. And an optically compensatory polarizing plate comprising a laminate of the above optically compensatory film and a polarizing plate is provided, wherein the phase retarder layer of said optically compensatory film is composed of a liquid-crystal polymer. Further, a liquid-crystal display device comprising a liquid-crystal cell is provided, wherein the above optically compensatory film or a combination of the optically compensatory film and a polarizing plate is disposed on at least one side of said liquid-crystal cell.
According to the first aspect of the present invention, there can be provided an optically compensatory film exhibiting birefringence characteristic hardly changed by an external stimulus such as heat and excellent in weight saving per unit area. The optically compensatory film can be adapted for reliably forming a liquid-crystal display device excellent in weight saving in spite of a large screen size, exhibiting a stable compensating effect, and excellent in uniformity of display quality to make visibility property good.
According to a second aspect of the present invention, there is provided an optically compensatory film comprising a transparent film base, and a birefringent phase retarder layer adhesively supported by said transparent film base, said transparent film base having a water absorption coefficient of not higher than 1.0% (at 23xc2x0 C. and for 24 hours) and having an photoelastic coefficient of not larger than 30xc3x9710xe2x88x9212 m2/N. And an optically compensatory polarizing plate comprising a laminate of the above optically compensatory film and a polarizing plate is provided, wherein the phase retarder layer of said optically compensatory film is composed of a liquid-crystal polymer. Further, a liquid-crystal display device comprising a liquid-crystal cell is provided, wherein the above optically compensatory film or a combination of the optically compensatory film and a polarizing plate is disposed on at least one side of said liquid-crystal cell.
According to the second aspect of the present invention, there can be obtained the same effect as that of the first aspect of the present invention. This is based on the fact in the second aspect that the coefficient of water absorption of a transparent film base in the optically compensatory film is controlled.
That is, the inventors of the present invention have made investigation and investigation eagerly to overcome the problem of the change of the birefringence characteristic. In the meantime, it has been found that the size of a cellulose polymer constituting a transparent film base in the background-art optically compensatory film varies widely in accordance with the change in quantity of water absorption and that the cause of the problem is the influence of the change of the size on the birefringence characteristic of the optically compensatory film. The inventors have succeeded in controlling the change of the birefringence characteristic by controlling the coefficient of water absorption of the transparent film base into the foregoing range.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.